The present invention builds on the invention disclosed in PCT/NZ01/00052. In this patent application, a two stage process is described for producing a biodegradable foamed product with improved packaging properties including resilience, compressibility and shock absorption. The entirety of PCT/NZ01/00052 is incorporated herein by reference.
It is an object of the present invention to produce a novel bio-degradable foamed product.
The field of starch based bio-degradable foamed materials is widely discussed in the prior art. A variety of products exist that attempt to produce bio-degradable foamed materials as discussed in PCT/NZ01/00052.
Extruded Starch Foams for Molded Shapes
Patent U.S. Pat. No. 5,730,824 (National Starch) utilizes extrusion to produce foam panels. These panels are then laminated together to form thick sheets, which can be wire cut to varying size and shapes. This process has limitations due to the expensive capital equipment required for manufacturing. As a result of the expensive equipment, the method necessitates shipping ‘air’ as the product can only be made in central locations. In addition the shapes are either very limited or costly because they have to be cut out of sheets instead of molded during the foaming process.
Another example, U.S. Pat. No. 5,801,207 (Novamont) relates to taking foamed starch pieces, placing them in a bag or within layers of sheeting and molding the pre-expanded peanuts in foam-in-place molds. The limitations of this method are that the foamed peanuts used to make the molds are very bulky and take up a lot of store space, and again increase expense through having to ship ‘air’ to the point of use instead of sending dense pellets that can be foamed at point of use. The method is also a complicated procedure for the end-user, as they have to fill and seal bags of foamed peanuts and then molds the bag to the product shape.
From the above it is hence useful to have a process that allows in situ foaming and further, that the equipment is relatively inexpensive and simple to use.
Microwaved Starch Foams for Molded Shapes
Two main patents, WO9851466 (Ato-Dlo) and U.S. Pat. No. 5,639,518 (NKK), utilize dielectric heating in processing the starch based materials.
In WO9851466 (Ato-Dlo), the dielectric heating does not take into account the changing dielectric properties of the material as it heats, nor the relationship between the rheological properties (for example elasticity and viscosity) and the rate of heating. It further doesn't identify or address the detrimental effect of vapor condensation on the foam surface finish when such a process is used.
Patent U.S. Pat. No. 5,639,518 (NKK), utilizes a number of different electromagnetic and electroconductive methods for producing foam bio-degradable shapes. It does not identify the importance of a rate of heating profile or specific rheology of the material being heated on the success of foaming thick walled bio-degradable shapes. It further does not identify or address the detrimental effect of vapor condensation on the foam surface finish when utilizing microwave frequency irradiation.
A further patent, WO 02/20238, (Ato B.V.), details a process of steam heating taking 5 minutes to heat, under pressure, to the desired temperature range of 185° C. Such a long processing time reduces throughput significantly for a semi-continuous process.
In addition, the methods described above produce foams with varying consistency depending on the shape required and, often without the combination of uniform physical and mechanical properties. These properties include density, compressibility, resilience and shock absorption. All of these properties limit the product applications. It is therefore desirable to have a method of processing that can produce a uniform product using equipment that is relatively inexpensive and simple to use.
Microwave Oven Designs
U.S. Pat. No. 4,908,486 (Nearctic Research Centre) describes a multiple magnetron microwave oven design where the oven is comprised of a cavity and at least one energy source. The main advantage disclosed of multiple energy sources is that the uniformity of drying is improved thus avoiding hot spots and cold spots inherent in some designs using only single energy sources. The oven is described as being useful for the drying of granular food crops including grain, rice, some fruits and beans. The apparatus does not however give consideration to use for foaming of materials, an object of the present invention. Further it does not consider the influence of adjusting the power density of the energy sources. The specification does not teach of processing multiple work pieces at any one time and further, does not address the use of molds, shapes and objects other than granular materials.
It is hence useful to have an apparatus that addresses aspects such as variable energy density, and complex multiple work pieces.
Surface Coatings
In attempts to improve the surface texture or colour of microwaved products, susceptors have been considered in a number of applications, especially in domestic food applications. Susceptors are typically metallic films attached to microwave packages which are used in food applications to crisp or brown the food surface.
For foamed bio-degradable materials, where the goal is to produce a soft, smooth finish rather than the crisp texture desired in food applications, problems have been experienced surrounding vapor condensation at the interface between the mold wall and the foamed material. Although this problem is identified for example in U.S. Pat. No. 5,965,080, (NKK), this US patent refers to the problem of vapor breaking down an insulating layer on conductive molds causing arcing, a processing problem specific to the use of conductive molds, rather than the effect of vapor condensation on the surface finish of the foam.
A further patent, U.S. Pat. No. 6,241,929 (Akopyan), recognises that uniformity is affected when the heat flow on the interface between the mold and the foamable material is large and teaches that it is necessary for the material and the mold to have much the same dielectric properties. The patent, whilst describing a principle behind vapor condensation and its effect on uniformity, does not teach of specific processes and examples, particularly for biodegradable foam applications.
It is therefore desirable to have a process utilizing susceptors that also produces a uniform product with a smooth surface finish.
Microwave Mold Designs
U.S. Pat. No. 5,965,080 (NKK) teaches of a method of foaming starch using conductive mold halves and an insulating section between. Both halves then have an alternating current applied thus heating and expanding the material. The importance of having vapor release sections is recognised as otherwise it is acknowledged that insulation breakdown occurs.
This method however has the problem that conductive molds have a limited rate of heating range as arcing occurs with increased power densities. Uniformity is a further problem with this method in that fringe effects occur in corner areas. Further, complex shapes, which include a mix of thin and thick walled foam, are difficult to make using this method as the method is limited by arcing that occurs in thin walled areas.
Two alternative mold arrangements have been considered for expanded plastic materials.
U.S. Pat. No. 4,298,324 (Isobox-Barbier) describes a device for molding expanded plastic material. The device consists of a press, a mold body and resonant cavity combination. Mold surfaces in contact with material being molded are formed from a resin containing carbon black, which has high dielectric losses, and the remaining portion of the mold body is made of a microwave transparent or transmissive material.
U.S. Pat. No. 5,397,225 (Huels) recognises the attributes of good dimensional accuracy and long serviceable lifetime for molds to form latex foams with microwaves. Limitations of practicable wall thicknesses of typical microwave transparent materials are discussed as are exposures to fluctuating temperatures. A new material based on polyphenylene ether with a passivated surface is described.
Whilst both methods describe useful alternatives, the limitations and constraints found from using bio-degradable materials are not considered.
Mold Liners
U.S. Pat. No. 5,508,498 (Invenetics) teaches of a utensil being a matrix material and a microwave absorptive material. The matrix is formed from silicone rubber with a ferrite based absorber material. The patent teaches only of use directed towards food applications and does not consider closed molds or pressure changes that occur within the mold.
U.S. Pat. No. 4,566,804 (CEM) discusses use of a supporting body for analysing a product where the supporting body is comprised of a matrix material and a microwave absorptive material, evenly dispersed within the matrix material, and is characterized by a Curie temperature of 120-140° C. The invention is limited to a purpose of analysing thermally sensitive materials for volatile components and does not contemplate foaming of a low dielectric material like starch resin within an enclosed mold.
U.S. Pat. No. 5,079,397 (Alcan) teaches of at least two regions of different lossiness in its susceptor materials. Examples of lossy substances suitable for inclusion in microwave susceptors are disclosed as well as techniques for application.
None of the above patents however account for use of a susceptor-type product with a closed mold for bio-degradable foams. In particular, they do not address the critical problems of susceptor and closed mold applications, being the prevention of condensation from of vapor released, and the internal pressures that accumulate within a mold during starch based foaming processes.
Thin Film
A large number of patents refer to the use of thin films as [a]susceptors. For example, U.S. Pat. No. 5,019,681 (Pillsbury) outlines prior art in the field of thin film susceptors where a thin layer such as polyester is used as the substrate with a thin metal film deposited on the substrate. U.S. Pat. No. 5,019,681 outlines further problems, specifically directed towards the breakdown of the susceptor during heating leaving it only suitable for disposable single-use applications.
The prior art whilst helpful does not identify applications requiring and detailing the constraints necessary for successful bio-degradable foam applications. In particular, the prior art does not address the issues inherent to susceptors used in conjunction with closed molds as described above. Namely, being the prevention of condensation from vapor released, while retaining a soft, smooth, surface finish, and the internal pressures that accumulate within a mold during starch based foaming processes.
Other Particulate Options
U.S. Pat. No. 5,294,763 (Minnesota Mining) describes particulate susceptors. Particulate susceptors can be divided into two categories; electrically continuous (e.g. carbon black) or electrically discontinuous (e.g. ferromagnetic particles).
Again the patent does not describe bio-degradable material foaming applications and hence does not consider the particular problems associated with these materials.
It is an object of the current invention to overcome the limitations of the methods above.
It is a further object of the present invention to produce a foamed product with uniform physical and mechanical properties such as density, compressibility, resilience, shock absorption and surface finish by addressing the combination of problems with rate of heating, heating method and mold design in combination.
It is a further object of the present invention to produce a foamed product that is bio-degradable and relatively inexpensive compared with previous methods.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.